This invention relates to a continuously variable transmission (CVT) such as a toroidal-type CVT, which is used to transmit rotation from an input side to an output side while continuously varying the speed by traction drive in vehicles such as automobiles, industrial machinery and so forth, and more particularly to an improved power roller assembly useable in the continuously variable transmission.
In general, the toroidal-type CVT includes an input disk on an input shaft, an output disk on an output shaft coaxial with the input shaft, and a pair of power roller assemblies adapted for transmitting motive power of the input shaft to the output shaft. The input and output disks are opposed to each other and have traction surfaces defining a toroidal cavity. The power roller assemblies are rotatably disposed within the toroidal cavity. Each power roller assembly includes rolling elements such as balls and a pair of races contacted with the rolling elements, one of the races having a traction surface contacted with the traction surfaces of the input and output disks. The race is rotatable about a first pivot positioned at the center thereof and pivotable about a second pivot positioned at the center of the toroidal cavity of the input and output disks when viewed in section taken along a common axis of the input and output shafts.
U.S. Pat. No. 5,556,348 discloses a toroidal-type CVT includes input and output disks and a power roller assembly disposed between the input and output disks. The power roller assembly includes a power roller engaging the input and output disks and acting as one of a pair of races of a ball bearing. The input and output disks and the power roller are subjected to surface-hardening and grinding so as to have effective carburized depths in a range of 2.0-4.0 mm for the purposes of improvement in rolling contact lives of traction surfaces of the input and output disks and power roller and in fatigue fracture lives of the input and output disks and power roller.
U.S. Pat. No. 5,510,974 discloses a grease-sealed bearing including rolling elements and rings having rolling surfaces which are in contact with the rolling elements and formed with iron oxide layers. This technique contemplates to eliminate the catalytic action of the rolling surface which will occur under high-temperature and high-pressure conditions during high-speed rotation of the rolling elements, and to suppress the chemical decomposition of grease used in the bearing which may be caused by the catalytic action. This attempts to prevent production of hydrogen infiltrating into a metal of the rings to cause embrittlement thereof. The iron oxide layers are formed by blackening treatment in which the rings are immersed in a caustic soda solution heated at 130xc2x0 C.-160xc2x0 C.
U.S. Pat. No. 6,051,080 discloses one of races of a power roller assembly for a toroidal-type CVT. The race is adapted to be in contact with input and output disks of the CVT and has a groove for receiving balls and forming a bearing surface. This technique contemplates to reduce the loss of a hardened layer caused by grinding conducted after surface-hardening of the race to thereby improve durability of the bearing surface of the race. The bearing surface has a carbon concentration not lower than that of a back surface opposed to the bearing surface. The bearing surface also has a hardened layer an effective depth of which is not smaller than that of the back surface.
When the conventional toroidal-type CVT is driven, a high load is applied to the traction surfaces of the input and output disks and the traction surfaces of the races of the power roller assemblies. This will cause a high contact pressure exerted on the bearing surfaces of the races of each power roller assembly which are in rolling contact with the rolling elements such as steel balls. At this time, the maximum contact pressure may be not less than 3 GPa. Further, unlike the case of usual ball-and-roller bearings, traction force and radial load are applied onto the bearing surfaces of the races of the power roller assembly when the rolling elements roll on the bearing surfaces. This may cause microscopic metal-to-metal contact between the bearing surfaces and the rolling elements or increase rolling-friction resistance generated therebetween, whereby tangential force applied onto the bearing surfaces will become large so that rolling-fatigue lives of the bearing surfaces will be lowered.
There is a demand for reducing the microscopic metal-to-metal contact and rolling-friction resistance caused between the rolling elements and the bearing surfaces of the races of the power roller assembly of the toroidal-type CVT to thereby improve the rolling-fatigue lives of the bearing surfaces thereof. In addition, the blackening treatment using the caustic soda solution as disclosed in the above-described conventional technique will make adverse influence on working environment and therefore it is industrially undesirable.
An object of the present invention is to provide a power roller assembly useable in a toroidal-type continuously variable transmission (CVT) which is improved in lubricating oil retention on bearing surfaces of a pair of races which are in contact with rolling elements, and capable of preventing the metal-to-metal contact and reducing rolling-friction resistance caused on the bearing surfaces upon operation of the toroidal-type CVT, then improving the rolling-fatigue lives of the bearing surfaces.
According to one aspect of the present invention, there is provided a power roller assembly for use in a continuously variable transmission, comprising:
a first race formed with a first bearing surface;
a second race spaced from the first race, the second race being formed with a second bearing surface opposed to the first bearing surface;
a plurality of rolling elements rolling between the first and second races, the rolling elements being in contact with the first and second bearing surfaces; and
an iron phosphate-based coat formed on at least one of the first and second bearing surfaces.
According to a further aspect of the present invention, there is provided a toroidal-type continuously variable transmission, comprising:
a pair of disks arranged in a coaxial and spaced relation to each other, the disks cooperating to form a toroidal cavity therebetween; and
a power roller assembly rotatably disposed within the toroidal cavity, the power roller assembly having a pivot positioned at a center of the toroidal cavity, the power roller assembly comprising:
first and second races formed with first and second bearing surfaces opposed to each other;
a plurality of rolling elements rotatable between the first and second races, the rolling elements being in contact with the first and second bearing surfaces, and
an iron phosphate-based coat formed on at least one of the first and second bearing surfaces.
According to a still further aspect of the present invention, there is provided a method of producing a power roller assembly for use in a toroidal-type continuously variable transmission, the power roller assembly comprising a pair of races formed with bearing surfaces, respectively, a plurality of rolling elements in contact with the bearing surfaces, and an iron phosphate-based coat formed on the bearing surface of at least one of the races, the method comprising:
subjecting a workpiece to forging and rough machining to form a preform;
subjecting the preform to surface-hardening;
subjecting the surface-hardened preform to grinding and superfinishing to provide the races formed with the bearing surfaces;
cleaning the bearing surface of at least one of the races with a cleaning agent solution; and
immersing the cleaned bearing surface in a surface-treatment agent solution containing phosphorus to form the iron phosphate-based coat on the bearing surface.